DBAL-522 - SQLParserUtils always throws exceptions on null bound params

Hotfix for [DBAL-522](http://www.doctrine-project.org/jira/browse/DBAL-522)

Demonstrates that NULL parameters are handled incorrectly by `Doctrine\DBAL\SqlParserUtils` as of 2.3.4.

Basically, following usage always throws an exception:

```php
$conn->executeQuery(
    'INSERT INTO FOO (foo, bar) values (:foo, :bar)', 
    array('foo' => 1, 'bar' => null)
);
```